Science Class 11 - Hydrocarbons Notes

Hydrocarbons

Welcome to the chapter on Hydrocarbons for Class 11. In this chapter, you will learn what hydrocarbons are, their types, properties, and uses. By the end of this chapter, you will understand the structure, classification, and reactions of hydrocarbons, and their importance in daily life and industry.

Introduction

Hydrocarbons are organic compounds made up of only carbon and hydrogen atoms. They are the simplest organic compounds and form the basis of organic chemistry.

Classification of Hydrocarbons

• Saturated Hydrocarbons (Alkanes): Contain only single bonds between carbon atoms. General formula: C_nH_2n+2. Example: Methane (CH₄), Ethane (C₂H₆).
• Unsaturated Hydrocarbons: Contain one or more double or triple bonds.
  • Alkenes: At least one double bond. General formula: C_nH_2n. Example: Ethene (C₂H₄).
  • Alkynes: At least one triple bond. General formula: C_nH_2n-2. Example: Ethyne (C₂H₂).
• Aromatic Hydrocarbons: Contain one or more benzene rings. Example: Benzene (C₆H₆).

Sources of Hydrocarbons

• Petroleum and natural gas are the main sources.
• Coal is also a source of aromatic hydrocarbons.

Properties of Hydrocarbons

• Generally non-polar and insoluble in water.
• Burn in air to produce carbon dioxide and water (combustion).
• Physical properties (boiling point, melting point) depend on molecular size and structure.

Important Reactions of Hydrocarbons

• Combustion: Hydrocarbons react with oxygen to give CO₂, H₂O, and energy.
• Substitution (Alkanes): Replacement of hydrogen by halogen (e.g., chlorination of methane).
• Addition (Alkenes/Alkynes): Addition of hydrogen, halogens, or water across double/triple bonds.
• Polymerization: Small alkene molecules join to form large polymers (e.g., ethene to polythene).

Uses of Hydrocarbons

• Fuels (LPG, CNG, petrol, diesel, kerosene).
• Raw materials for plastics, synthetic fibers, and chemicals.
• Solvents and industrial chemicals.

Practice Questions

1. What is the general formula for alkanes?
2. Name one saturated and one unsaturated hydrocarbon.
3. What happens when methane burns in air?
4. Give an example of an aromatic hydrocarbon.
5. What is polymerization? Give one example.

Challenge Yourself

• Draw the structure of ethene and ethyne.
• Explain the difference between addition and substitution reactions with examples.

Did You Know?

• Benzene was first discovered in coal tar.
• Natural gas is mainly methane, the simplest hydrocarbon.

Glossary

• Hydrocarbon: Compound made of only carbon and hydrogen.
• Saturated: Contains only single bonds.
• Unsaturated: Contains double or triple bonds.
• Aromatic: Contains benzene ring(s).
• Polymerization: Process of joining small molecules to form a large molecule.

Answers to Practice Questions

1. C_nH_2n+2
2. Saturated: Methane (CH₄); Unsaturated: Ethene (C₂H₄)
3. It produces carbon dioxide, water, and energy (heat and light).
4. Benzene (C₆H₆).
5. Polymerization is the joining of small molecules (monomers) to form a large molecule (polymer), e.g., ethene to polythene.

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Understanding hydrocarbons is the foundation of organic chemistry and helps us in many fields like energy, industry, and environment!